Downloads always corrupted or data missing...

Recently my system was formatted and xp pro re-installed.....afterwards firefox had a problem of the page not loading...i fixed that by installing nvidia network manager.....but now another weird problem has arisen.....whenever i download something ,be it videos,programs etc. they are corrupt or incomplete, the computer says they are missing information or data etc. I thought it was a problem with firefox,so i installed flashget from an old cd...then it said an update was available...so i downloaded the latest flashget from the site....and when i tried to install it, it said 'data missing'....i tried every program and download manager i have on CDs......orbit,DAP,Opera and they download the files perfectly...but the downloaded file is corrupt or missing data......but......my p2p works perfectly...weird....